The present invention relates to an apparatus for applying a load to a strain gage transducer beam, especially in a so-called platform scale. Such scales have a weight capacity in the range of 30 to 1000 lbs.
Typical examples of such platform scales are disclosed in the above mentioned U.S. Pat. No. 4,261,429. The bending beam transducers employed in such platform scales preferably form integral components of the platform proper. For this purpose slots are milled or machined or cut into the platform so that two parallel slots define a beam to which the strain gage elements are secured. The sensitivity of the transducer beams is adjusted by drilling holes into the platform at each inner end of each slot and between the ends of each slot. The free end of each beam is operatively supported on a rubber leg. Overload limiting elements are so arranged that the flexing of each beam is limited to a predetermined value.
Loads applied to the platform or to an intermediate member cause deflections of the sensing or transducer beams and these deflections in turn reduce the axial length of the beam. The length reduction is taken up by the resilience or compliance of the rubber legs. However, since the resilience or compliance is low, the application of horizontal load components to the sensing beams cannot be entirely avoided. Further, the platform is also subject to deflection in the vertical direction causing further shortening in both the longitudinal and in the transverse directions. Since platform deflections and resultant shortening in the longitudinal direction cause additional horizontal load components to be are applied to the sensing means. The same holds true for shortening of the platform in the transverse direction. In this connection the transverse and longitudinal directions of the platform are directions extending in the horizontal plane of the platform.
The above mentioned horizontal load components are undesirable because they cause nonlinearity errors in the output of the scale as well as higher hysteresis due to the poor mechanical characteristics of the rubber legs. Additionally, the so-called creep performance and the nonreturn to zero of the output display of the scale may also be attributed to the poor mechanical characteristics of the rubber legs as employed heretofore.
In addition to the above mentioned undesirable effects caused by longitudinal and lateral deflections, side loads applied to the scale during its operation may have similar effects. For example, if in addition to a load already on the scale a side force is inadvertently applied to the loaded scale some slippage may occur between the rubber feet or legs and the supporting surface. These side loads change the above mentioned effects resulting from horizontal load components or so-called "locked in forces" whereby the scale output is altered. Further output changes may be caused when the load to be weighed is applied in a direction not exactly vertical.
The above adverse effects primarily caused by the rubber feet have resulted in a limitation of the inherent and desirable performance characteristics of platform scales of the above described type having the sensing beams constructed as an integral component of the platform. Such platform scales are also referred to as "gaged plate" scales. Thus, the accuracy of such scales has been limited heretofore to approximately 0.05%. While such accuracy is quite adequate for many purposes, there is room for improvement, particularly in the application of the load to the sensing beam proper so as to avoid substantially all adverse load components.